He keeps on accumulating endless unhappiness (5)
I believe the answer is:
miser
'he keeps on accumulating' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'endless unhappiness' is the wordplay.
'endless' means to remove the last letter.
'unhappiness' becomes 'misery' (misery is a kind of unhappiness).
'misery' with its final letter removed is 'MISER'.
Can you help me to learn more?
(Other definitions for miser that I've seen before include "Stingy hoarder" , "One of the people who spend least" , "Mean-minded type" , "Skinflint, like Scrooge" , "Penny-pincher" .)